The invention relates to a method and an arrangement in connection with an inverter. The invention relates particularly to controlling an inverter consisting of a plurality of sub-inverters, each being connected by a specific cable to feed a common load.
The power stages of power inverters intended for the low-voltage range are usually composed of a plurality of parallel IGBT switch modules. Conventionally, the parallel connection is performed phase-specifically by rigidly interconnecting the switch module poles intended for feeding the same phase in the immediate vicinity of the modules, and by connecting the motor cable conductors of the same phase to said interconnection by means of suitable busbar arrangements.
However, this procedure has a plurality of generally known drawbacks. In order for the phase current to be divided evenly between the rigidly parallel-connected switch module branches, the modules have to be selected to be as similar as possible relative to the voltage loss of the IGBT switches. This increases costs and complicates spare part service. However, matching the voltage loss of the IGBT switches does not usually mean that the voltage losses of the diodes parallel to the switches would be correspondingly matched; instead, extremely significant differences may exist, which may lead to some diodes being overheated.
Each parallel-connected IGBT module has a specific rise time and fall time, which cannot be matched when the voltage losses of the switches are matched. This may cause situations wherein the fastest module switches on the entire phase current and, correspondingly, the slowest module switches off the entire phase current. This results in a serious increase in switching losses and heating at these modules.
As components comprising three switch branches (six-pack) are generally used in the parallel connection of phase modules, each of the branches being composed of e.g. three IGBT chips parallel-connected inside the module, a situation may arise even at average powers wherein there are 9 to 12 rigidly parallel-connected chips. A generally used technique is to indicate any breakthrough of a phase branch by measuring the saturation voltage, i.e. the voltage of either the lower or the upper IGBT chip array against the minus or, correspondingly, the plus busbar. However, this technique does not work if there are many parallel-connected chips, since the short-circuit current of a defectively working chip is known not to be sufficient to draw correctly working chips off saturation; instead, the defectively working module explodes.
Attempts are known to have been made to prevent the above problems with a switch module-specific output inductor, wherein the switch components are connected in parallel after the output inductor. However, such inductors are bulky, they cause much dissipation power and are expensive.
A rigid parallel connection also means that at high powers more than two parallel-connected motor cable phase conductors may be required. This is not possible in the electrical safety prescriptions of several countries without a special permit; instead, in most cases, the requirement is that each of the rigidly interconnected sub-conductors, too, is protected against short-circuit and overload current either with a fuse, a relay function or current measurement. In some cases, this is required at both ends of the cable. These protective components are often expensive and require much space.
The above-described problem of parallel connection is known to have been minimized by connecting each of the parallel-operating three-phase frequency converters with a specific cable directly to the motor in such a manner that the corresponding phases of the parallel frequency converters are not connected until at the motor conduit box. Such a solution allows all the above-described parallel operation problems to be solved, provided that the output currents of each converter are measured and used for protecting the motor cables against short-circuit and overload.
However, the latter method does not either eliminate the disadvantage that may result from the extremely fast IGBT switches switching their states in the coils and bearings of the motors. Because of the switching, a surge wave typically propagates to the coil, the rate of change of the leading edge of the wave being several kilovolts per microsecond. In addition, the reflection phenomenon caused by the discontinuity point generated by the unequal wave impedances of the cables and the motor may, in the worst case, almost double the current peak appearing in the motor poles compared with the voltage level of the intermediate circuit outgoing from the inverter. Such a voltage steepness and large amplitude are known to cause such high turn voltages, particularly in unfavorably coiled wire coils, that there exists the possibility of a partial discharge. An extendedly acting partial discharge may finally damage the insulation of the winding wire, and the resulting turn short-circuit destroys the coil.
The surge wave propagating in the motor cables also causes capacitive polyphase currents via the ground potential, which puncture the lubricant membrane when passing through the motor bearings, thus shortening their service life.
Conventionally, the steepness of the edge of the surge wave caused by the IGBT output stage is lowered by using either output inductors or a specific LC output filter. However, these components are bulky and expensive.